Condensation is a common separation mechanism for many industrial processes. It is typically used with a heat transfer surface that is controlled at a temperature that will remove specific vapors from a gas stream. When vapors condense on these surfaces they form a combination of a liquid film, rivulets, or droplets that drain by gravity to the lowest point of the surface and then typically fall from the surface in droplet form. In most cases the heat transfer surface must be remote from the original high concentration source of the vapor to prevent the droplets from being reintroduced to the process. Locating the surface at a distance from the original source generally requires that the condensing operation occurs at a lower vapor concentration. This leads to inefficiencies that require colder operating temperatures, more surface area, or reduced rates of condensation.
In some processes, active systems remove the condensed liquid from the surface. For example a rotating surface with a scraping blade to remove the condensed liquid film. These systems can be located close to the process since the liquid is removed by mechanical means and will not be reintroduced to the process. However, these systems are complex, expensive, and have limited design flexibility to fit a particular process.
One example of a component separation system are the conventional dryers of coated substrates which direct large volumes of gas to the coated surface to evaporate the liquid and remove it in vapor form. In many cases, due to environmental or economic reasons, the vapors in the gas stream must be removed before exhausting to the atmosphere. Condensation is a common method of removing these vapors from the gas stream.
Systems using condensation typically are large heat exchangers or chilled rolls with wiping blades. They are located away from the web in-the bulk gas flow stream. Typically a heat exchanger is placed in the bulk gas flow stream and the surface temperature is reduced to a point where the vapors condense, removing them from the process.
The vapor concentration in the bulk gas flow stream must be kept below flammable limits, typically 1-2% of the gas stream. To obtain acceptable recovery efficiencies with low vapor concentration, the heat transfer surface must be large and the operating temperatures must be very low, on the order of -30.degree. C. -0.degree. C. This is very expensive and there are numerous process problems such as ice formation on the heat transfer surfaces. Significant effort has been expended in raising the vapor concentration to improve the efficiency of this process. An example is the use of inert gas as the gas flow stream which allows the vapor concentration to be increased, because the flammability limit is removed. However, these methods are also very expensive and create additional process problems that have limited their use in the industry. The vapor concentration at the surface of the coating, or gas/liquid interface, is very high and it drops off rapidly within several centimeters of the surface.
It would be ideal for maximum recovery efficiency to locate the heat transfer surface within several centimeters of the gas/liquid interface. Condensing surfaces generally cannot be located this close to the surface as the condensed liquid would drain by gravity back onto the coating surface. Active systems such as a rotating roll with a scraping blade could be close to the process but their shape does not fit the flat surface characteristic of the moving substrate and they are relatively complicated and expensive. There is a need for a system of removing the liquid from condensing surfaces in a component separation system.